An infrared laser having a wavelength of 850 nm or 940 nm is used in fields such as optical communications, time-of-flight (TOF) measurement, and on-vehicle imaging laser radars (also called SPAD LIDAR (Single Photon Avalanche Diode Light Detection And Ranging)). This is because, if irradiation light is visible light, there arises a problem that a malfunction may generate due to natural light (visible light) present in environments, or that the irradiation light is visually recognizable and the device operation is annoying to users. On the other hand, infrared light does not cause such a problem and is safer to the eyes because light energy is low. For that reason, the infrared laser having the wavelength of 850 nm or 940 nm is used as described above. In the above-mentioned circumstance, an avalanche photodiode with high efficiency of photon detection has attracted attention as a light receiving element that detects weak infrared light of 850 nm or 940 nm at high speed.
When a reverse bias voltage lower than the breakdown voltage is applied, the avalanche photodiode operates in a linear mode, and an output current varies depending on an amount of received light with a positive correlation. On the other hand, when a reverse bias voltage equal to or higher than the breakdown voltage is applied, the avalanche photodiode operates in a Geiger mode. Because the avalanche photodiode in the Geiger mode causes the avalanche phenomenon (avalanche amplification effect) even with incidence of one photon, a large output current is obtained. Therefore, the avalanche photodiode in the Geiger mode is also called a SPAD (Single Photon Avalanche Diode). In the Geiger mode, the avalanche photodiode is controlled such that, with application of a voltage between a cathode and an anode, electric field intensity of 3.0×105 V/cm or higher is generated at a junction where the avalanche phenomenon occurs. Charges generated by weak light near the junction are amplified under the above-mentioned control. Thus, the avalanche photodiode can react with a very small input signal corresponding to one photon and can output an electric signal. Furthermore, the avalanche photodiode reacts with an optical signal in a very short time of about several picoseconds and has high time resolution.
Under the above-described situations, the applicant has previously proposed, as one type of SPAD, a SPAD (entirety of which is denoted by reference sign 100) having a structure illustrated in FIG. 8 and being integral with a CMOS (Complementary Metal Oxide Semiconductor) (see Japanese Patent Application No. 2015-235026). The SPAD 100 includes an N-diffusion layer 111 formed inside a P-type substrate (silicon) 110 and spreading in a lateral direction, a P-well 112 (depth of about 0.5 μm) formed on the N-diffusion layer 111 in contact therewith, a P+-layer 113 formed in a shallow portion of the P-well 112, an N-well 115 formed inside the P-type substrate 110 in contact with the N-diffusion layer 111 at a position away from the P-well 112 in the lateral direction, and an N+-layer 116 formed in a shallow portion of the N-well 115. The N-well 115 is arranged inside the P-type substrate 110 to surround the P-well 112 from opposite sides spaced in the lateral direction (see FIG. 9). A region between the N-well 115 and the P-well 112 inside the P-type substrate 110 is an N−−-region 114 having a low concentration. A region of STI (Shallow Trench Isolation) 121 made of a buried oxide film for element isolation is formed along each of both sides (inner and outer peripheral sides) of a surface portion of the N-well 115. On the P+-layer 113 and the N+-layer 116, an anode contact electrode (aluminum) 131 and a cathode contact electrode (aluminum) 132 are disposed respectively in ohmic contact with them. Moreover, a silicon oxide film (SiO2) 122, a polysilicon electrode 133, a silicon nitride film (Si3N4) 123, an interlayer insulating film 124, a light shield film (AlCu) 125, a silicon oxide film (SiO2) 126, and a silicon nitride film (Si3N4) 127 are successively laminated on the P-type substrate 110 in the mentioned order.
In the SPAD 100, as schematically illustrated in FIG. 9, the avalanche phenomenon is generated by applying a high electric field to a junction AJ between the N-diffusion layer 111 and the P-well 112. More specifically, a signal line is connected to the anode contact electrode 131, and a constant voltage (e.g., about 10 to 30 V) is applied to the cathode contact electrode 132. The P-type substrate 110 is held at a ground potential. Thus, voltages are applied between the N-diffusion layer 111 and the P-well 112 and between the N-diffusion layer 111 and the P-type substrate 110. At that time, between the N-diffusion layer 111 and the P-well 112, because the P+-layer 113 serves as a stopper for a depletion layer and a width of the depletion layer is limited, the electric field intensity is increased in comparison with that between the N-diffusion layer 111 and the P-type substrate 110 (i.e., near a junction NJ), namely to 3×105 [V/cm] or higher. Accordingly, the avalanche phenomenon occurs in a region between the N-diffusion layer 111 and the P-well 112 (i.e., near the junction AJ), and the relevant region functions as an avalanche amplification layer. In other words, when a photon PH is incident on the SPAD 100, the P-well 112 functions as a light detection region and generates a carrier CR by photoelectric conversion. The carrier CR is attracted by the electric field and is drifted to the vicinity of the junction AJ. Because the electric field of 3×105 [V/cm] or higher is applied to the junction AJ as described above, the carrier CR is amplified 10,000 times or more by avalanche amplification. As a result, a large output signal can be taken out even with incidence of one photon.